This invention relates to a brake hydraulic pressure generator which has a control valve and generates hydraulic pressure according to manual braking effort applied through a brake pedal, particularly to a brake hydraulic pressure generator in which the brake operating stroke is substantially not influenced by the amount of brake fluid consumed in the wheel brake cylinders.
In prior art brake hydraulic pressure generators, the manual brake operating force is amplified by a booster and applied to a master cylinder. In this type of devices, the brake pedal stroke corresponds directly to the amount of brake fluid discharged from the master cylinder. Thus, the brake pedal stroke is inevitably influenced when an additional hydraulic device such as an antilock brake unit is activated.
The JP patent publications 2002-173016 and 59-109453 disclose brake hydraulic pressure generators that aim to solve this problem.
Publication 2002-173016 discloses various brake hydraulic pressure generators which can keep the brake pedal stroke from varying with change in the amount of brake fluid consumed in the wheel brakes. One of them uses a negative pressure as power source.
FIG. 5 shows a device which is the same as the device shown in FIG. 6 of the publication 2002-173016.
The stroke of an input shaft 4 of the device shown in FIG. 5 is substantially equal to the stroke of a piston 5b which is axially slidable relative to a power plate 15a. The pressure in a dynamic pressure chamber 15b pushes the piston 5b leftwardly in the figure against the force of a spring 7. The piston 5b stops at a point where the pressure in the dynamic pressure chamber 15b balances with the force of the spring 7. Since the stroke of the input shaft 4 is substantially equal to that of the piston 5b, the stroke of the input shaft 4 is determined by the pressure in the dynamic pressure chamber 15b. 
On the other hand, the pressure in the master cylinder 16 acts on the end 18 of the input shaft 4 as a reaction force against the pedal operating force. The pressure in the master cylinder 16 corresponds to the pressure in the dynamic pressure chamber 15b. Thus, the relation between the pedal stroke and the pedal reaction force can be set substantially independently of the amount of brake fluid consumed in the wheel brakes.
In these devices, the force of the spring 7 is determined such that the stroke of the master cylinder 16 is greater than that of the input shaft 4.
In the arrangement of FIG. 5, negative pressure is produced in a chamber 15c. The master cylinder pressure corresponds to the differential pressure between the chambers 15c and 15b until the differential pressure reaches its maximum. The differential pressure reaches its maximum when the pressure in the dynamic pressure chamber 15b is equal to the atmospheric pressure because the pressure in the dynamic pressure chamber never exceeds the atmospheric pressure. If the brake pedal is depressed with an increased force after the pressure differential between the chambers 15b and 15c reaches its maximum, the input shaft 4 is further pushed into the master cylinder 16 (moved leftwardly in the figure), while the master cylinder piston 16a is moved rightwardly in the figure against the differential pressure until the input shaft 4 abuts the master cylinder piston 16a. The master cylinder pressure and the reaction force applied to the input shaft 4 remain unchanged all the while. This means that the brake pressure cannot be increased any further until the input shaft 4 abuts the master cylinder piston 16a. This makes the driver very uncomfortable.
Particularly if the driver depresses the brake pedal with a force greater than the maximum pressure differential between the chambers 15b and 15c, the driver may suspect possible leak of brake fluid through pipes because the reaction force never increases even though the brake pedal is moving.
In the publication 59-109453, means are provided for checking the stroke of the input shaft before the pressure differential between the negative pressure chamber (e.g. chamber 15c) and the dynamic pressure chamber (e.g. chamber 15b) reaches its maximum. But in this arrangement, it is absolutely impossible to increase the brake pressure above the maximum differential pressure. This arrangement is not practically feasible because the negative pressure produced in the negative pressure chamber fluctuates, so that the maximum differential pressure, which directly corresponds to the negative pressure, also fluctuates. This means that the maximum braking force also fluctuates. This arrangement is not desirable to a brake hydraulic pressure generator that does not use negative pressure, either, because the means for checking the stroke of the input shaft makes it impossible to increase the brake pressure above the inherent maximum brake pressure even if much higher braking force is needed due to fading of friction materials.
An object of this invention is to provide a brake hydraulic pressure generator which does not allow the brake pedal to be pressed in without any increase in the reaction force against the pedal operating force.